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IJRA Vol. 1, No. 4, December 2012 : 203 213

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Yun et al. [5] have developed a transmuting track wheelchair which has a dynamic track tension

characteristic. They have used fuzzy logic control as an optimal estimation algorithm to estimate the

proportion of various factors that affect the track tension. Frame of the wheelchair consists of fixed shelf, seat

and track transmutation mechanism in which the main factors affecting tension are determined by fuzzy

decision. However, vibration occurs when the wheelchair gesture is adjusted, thus another controller needs to

be designed to make the main factors steadily follow appropriate value in order to avoid oscillation.Some disadvantages of these crawler systems are that the entire track is forced to rotate on the edge of the

first step when initiating a descent and low locomotion efficiency in barrier free environments [6].

B. Leg typeSugahara et al. [7]described the means of tuning-up method of walking parameters for a biped robot

with leg mechanisms using Stewart Platforms with the rise of 250 mm and certain walking experiments for

ascending and descending stairs carrying a human. The stroke range used could be reduced by tuning up the

waist yaw and preset zero moment point (ZMP) trajectories for motion pattern generation. Through certain

simulations, it was also confirmed that the maximum rise of the stair that WL-16RII could traverse was

250mm.

Solution based on the leg type have the highest adaptivity to rough terrain and can move on stairs ora slope with stability, since the contact points with the ground where the feet support the body can be selected

safely, but these mechanisms experience certain problems due to load weight, energy efficiency and speed ofmovement, and thus are not suitable as means of providing mobility for the elderly or the disabled.[6].

C. Hybrid typeLawn et al.[8] utilized a hybrid of four robotic legs actuated hydraulically and equipped with

independently operated steering and drive motors during stair climbing.

Morales et al.[6] designed a hybrid two decoupled wheelchair mechanisms in each axle; one to

negotiate steps and the other to position the axle with regard to the chair in order to accommodate the overall

slope. Kinematic model and trajectory planner were utilized to improve the trajectory planning on complex

notation. This has promised high percentage of time reduction in the climbing /descent process by using

optimized trajectory planner algorithm (Sliding mode control). However, this method requires more

complicated stabilization process and increased power consumption because high computation resources are

used in the control algorithms.

Young et al.[9] presented a 7 degree of freedom two-legs stair climbing wheelchair with laserdistance sensors to measure the heights and widths of the stairs and wheelchair direction error. The proposed

wheelchair can climb steep stairs (more than 30 degrees slope), while maintaining the seat trajectory

statically stable at all times. However, there is no stability control incorporated to ensure the stability of the

wheelchair.

D. Wheeled typeLawn et al.[10] developed front and rear wheel clusters connected to the base of the wheelchair via

powered linkages for high single step capability. This mechanism allows the wheelchair to climb up and

down the stairs as well as enter directly into and from a van. A minimum control system based on two single

chip computers and RC servo MUX and RC servo controllers have been embedded to operate the model.

However this method required too complex structure with eight wheels mounted at the base of the wheelchair

and may be perceived as a little too robotic, thus not suitable for indoor purposes.

Quaglia et al.[11] introduced mechanical concept for a stair climbing mechanism whichincorporated a four-bar linkage with twelve wheels on both right and left sides that can generate a relative

motion between the frame and the seat. Two functioning modes of operation were used for each locomotion

unit from rolling on wheels to stepping on legs without any command, but only based on surface and

dynamic conditions. However, this configuration gives unstable condition at the middle of the stairs and

requires a lot of energy during steering operation because of the local skidding between wheels and ground.

Sugahara et al.[12] proposed transformable wheeled four-bar linkages, which can transform fromparallelogram mode to straight and dogleg mode, for to TBW-1 Matsushima to complete the ascending and

descending stair climbing task. This method ensures a lower ground contact pressure and wide support

polygon than iBOT because all the four wheels are in contact with the treads at all times. However, the

stability control and sensing method for motion planning have not been studied and these are just based on

the inverse kinematic derivation and manually setting of joint angle transition to traverse stairs.

Teruaki I [13] designed a solid model for self-propelled stair climbing wheelchair using Computer

Aided Design software tool to validate the feasibility of the approach. The work has focus on the process-oriented approach only to help the user to achieve goal using intensive simulation-based study. However, no

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physical modeling has been used to validate the simulation as well as the safety measures and no controlalgorithm implemented.

Chun-Ta et al.[14] reported on the usage of rotational multi-limbed structures, which were mounted

pivotally on the opposite sides of the base. The author made use of short arm, long arm and support triangle

actuated with rotational multi-limbed structure in order to rotate through epicyclic gear trains to ensure the

stability of the wheelchair. However the project has not progressed to proper control method for stabilitypurposes and has used more motors and complex mechanism, hence is not suitable to use in domesticenvironments.

2. STAIR CLIMBING WHEELCHAIR ON TWO WHEELSThere are many studies conducted in areas related to wheel chair on two wheels. For example they

have focused on step climbing while the wheelchair is in inverted pendulum condition. Takahashi et al.[15]have proposed a rear wheel shaft position movement control scheme in order to raise the front wheels by

using small force when climbing about 75mm step at an inverted pendulum position. The optimal design of

rear wheel shaft position has been conducted to cater for instability of the wheelchair when the wheel shaft is

just under the centre of gravity. Proportional integral controller has been successfully implemented in order

to maintain the inverted pendulum condition in the presence of an impact and to recover small inclination

from backward and forward body position movement. Moreover, Takahashi et al.[ [16] have provideddetailed wheelchair modelling during inverse pendulum control of the front wheel raising while Takahashi et

al.[ [17] have shown the experimental result of step climbing using power assist wheelchair. A new scheme

for front wheel raising called back and forward moving scheme in which the wheelchair goes back first and

goes forward was introduced in [18]. A further scheme has been reported by Takahashi and co-workers

[19][20] in which to move the seat instead of moving the rear wheel shaft. The seat is moved slowly and the

desired chair body inclination is increased depending on the seat movement. All the methods discussed have

been realized with PI control until Takahashi et al.[ [21] decided to improve the performance with an

observer based optimal control (LQG or H2) with an added separate integral action. The control approach has

been tested and has shown better performance than PI control.

A noteworthy feature of the iBOT 3000 Transporter is that it operates on an articulated wheel

clusters design [22]. Stair climbing is achieved by controlling the cluster rotation on the basis of the position

of the centre of gravity (CG), whether operated by the user or an assistant. The device strives to keep the CG

of the system above the ground-contacting and between the front and rear wheels at all times, regardless ofdisturbances and forces operating on the system. The control system requires the device to dwell on each step

for a few seconds before allowing another cluster operation. This hiatus helps keep the device from running

away on stairs [22]. Each cluster may rotate about its central axis while the wheels may rotate about their

hubs.

However, this mechanism needs user to face down the staircase and always hold the handrail or

require an assistant to generate the control signal by causing a center of gravity shift as these may limit the

wheelchair to broken arms user. The seat height is too high to transfer to and from the iBOT, and in driving

the device in standard function within close quarters, thus not suitable for indoor purposes [1].

Stability for the iBOT wheelchair seat has not been taken into consideration during stair climbingprocess as the user must grip the handrails all the times to make sure that the seat is in the upright position.

The wheelchair prototype presented in this paper maintains the same behavior as the commercial

iBOT, with the addition of important property such as a capability to stabilize the wheelchair seat during

ascending and descending staircases. A wheelchair using cluster wheels is developed in MSC Visual Nastran4D software which mimics real system and can be characterized as a highly nonlinear, complex and unstable

system. This mechanism is quite simple and thus can be used for indoor purposes without the user needing tohold the guardrails all the times or assistants. In addition, the wheelchair seat is kept stable during the stairs

ascending process and the user needs not to face down the stairs.

3. SYSTEM MODELA new modified and simplified version of wheelchair model using cluster mechanism is designed

using MSC Visual Nastran 4D (VN) design software [23] as shown in Fig. 1. The VN software environment

can provide a visualization showing the stair ascending and descending process, stabilizing and other

functional features. It allows performing design and simulation of rigid body dynamics, determining part

interference and collision responses, identifying stresses induced by motion, producing physics-based

animation as well as control testing [24]. It provides a wide range of modeling and analysis capabilities,including linear static, displacement, stress, strain, vibration and heat transfer. In addition, it can easily be

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integrated with Matlab/Simulink for developing and testing controllers. The gravitational force is taken into

account by Visual Nastran, thus approximating the real system. Furthermore, it saves time and money

because if any changes are required, they can easily be tested in the software first before the actual system is

developed.

The wheelchair was modeled in a basic form comprise in a two pairs of cluster wheels, frames and

axes connected to the seat as shown in Fig 1.. Three pairs of motor are required in order to control theclimbing and stabilizing tasks;

Fand

R represent torques applied at the front and rear wheels respectively

while S represents the torque for the tilt angle. Link 1 is used to cater for the whole weight of the human

body while Link 2, which is located at the centre of the axle, is to cater for the seat and battery weights. The

sensors are attached at the respective reference bodies for control and measurement.

The humanoid model is designed and approximated using the anthropometric data based on

Winters work [25]. In this research, the humanoid model is developed as a rigid body with 1.5 m in heightand the weight of 71 kg. The dimensions and specifications of the wheelchair model are shown in Table 1.

Figure 1. Complete wheelchair model

Table 1. Dimensions and specifications of the wheelchair model

Body segment Dimension (m) Mass (kg)

Wheel 0.18 x 0.07 3

Seat 0.45 x 0.43 x 0.08 2Back rest 0.02 x 0.4 x 0.45 1.35

Front horizontal axis 0.04 x 0.55 1

Back horizontal axis 0.04 x 0.55 1

Base link 0.38 x 0.06 x 0.04 2

Left connecting rod 0.34 x 0.02 x 0.01 1.5

Right connecting rod 0.34 x 0.02 x 0.01 1.5Left base joint 0.05 x 0.02 1.58

Right base joint 0.05 x 0.02 1.58Vertical rod 0.03 x 0.59 3.03Battery 0.38 x 0.23 x 0.32 2

The VN environment can easily be integrated with Matlab for control design purposes by installing

the VN model as a plant into the Simulink library, where the VN icon can then be used in the Simulink

environment. Simulink in Matlab is used as a platform for control purposes thus each time the simulation is

active, the wheelchair system designed in VN is also receiving control signals and giving the system outputs.

Matlab is a well known software package for modeling, simulation, dynamics system analysis, continuousand discrete time analyses, and it supports linear and nonlinear system types. In the VN icon, the user has to

specify the input and output parameters to be controlled or measured as shown in Fig. 2. PID control is

adopted in this work for controlling the movement of the wheel and tilt angle of the wheelchair during both

ascending and descending staircases, and the tuning parameters were selected by using heuristically method.

F

S

R

Link 1

Link 2

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Figure 2. Control System Structure

4. SIMULATION AND RESULTSA. Ascending stairs

The proposed control approach was implemented in Simulink with VN for evaluating the

effectiveness of the control method for ascending step 1 and step2. The control objective is to produce

sufficient torque for lifting the rear wheels over the front wheels using cluster mechanism while maintaining

the human body on seat at the upright position. For this particular dimensions and specifications of

wheelchair model, the simulation in Visual Nastran was conducted on the specific stairs. Each stair had adepth of 0.28 m, height of 0.285 m and width of 1.81 m. Fig. 3 shows the movement of the wheelchair in VN

environment while ascending the stairs. As noted does not need to hold the handrails and face backwards

while climbing up the stairs. The controller provides sufficient torque at the front wheels in order to lift the

rear wheels over the other wheels. When the motor at the front wheels work, it will automatically lock the

front wheels and unlock the rear wheels to prevent the wheelchair from slipping away. The mechanismrepeats the same process with climbing up the second step. Fig. 4 shows the simulation results for climbing

up the first step. The PID controller maintains the tilt angle to zero degree position in less than 2.5 s as can be

seen in Fig. 4 (a). Fig. 4 (b) shows the orientation of the wheelchair base link which is set to the desired

angle depending on the stairs specifications. It can be clearly seen that the climbing process was completedin 1s and the performances of the wheels and seat torques are shown in Fig. 4. In order to climb up the

second step, the motors require higher torques to support the human and wheelchair weights as well as tomaintain the tilt angle at zero degree position as shown in Fig. 5. As noted the PID control was able to

control the movement of the rear wheels so that they can perform smoothly in 1 s.

(a) (a)

(b) (b)

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(b) Angular position of link

(c) Control torques

Figure 4. Wheelchair performance during ascending step 1

A. Tilt angle

0 0.5 1 1.5 2 2.5 3-100

-50

0

50

Time (s)

Angularpositionoflink(degree)

0 0.5 1 1.5 2 2.5 3-150

-100

-50

0

50

100

150

200

250

300

Time (s)

Torque(Nm)

Seat Torque

Wheel Torque

0 0.5 1 1.5 2 2.5 3-4

-3

-2

-1

0

1

2

Time (s)

Tiltangle(degree)

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B. Angular position of link

C. Control torques

Figure 5. Wheelchair performance during ascending step 2

(a) Tilt angle

0 0.5 1 1.5 2 2.5 3-60

-40

-20

0

20

40

60

80

100

120

140

Time (s)

Angularpositionoflink(degree)

0 0.5 1 1.5 2 2.5 3-400

-300

-200

-100

0

100

200

300

400

500

Time (s)

Torque(Nm)

Seat torque

Wheel torque

0 0.5 1 1.5 2 2.5 3-2

-1

0

1

2

3

4

5

6

7

Time (s)

Tiltangle(degree)

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(c) Control torques

Figure 7. Wheelchair performance during descending step 2

(a) Tilt angle

0 0.5 1 1.5 2 2.5 3-60

-40

-20

0

20

40

60

80

100

120

140

Time (s)

Angularpositionoflink(degre

e)

0 0.5 1 1.5 2 2.5 3-800

-600

-400

-200

0

200

400

Time (s)

Torque(Nm)

Seat torque

Wheel torque

0 0.5 1 1.5 2 2.5 3-2

-1

0

1

2

3

4

5

Time (s)

Tiltangle(degree)

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(c) Control torques

Figure 8. Wheelchair performance during descending step 1

B. Descending stairsFig. 6 shows the descending stairs process for the wheelchair while maintaining the user at the

upright position. The same method with ascending stairs was implemented here but at this time, the user

faces down the stairs. Fig. 7 shows the performance of the wheelchair during climbing down the second step.As noted, it required less than 1s to perform the descending task as compared to ascending task as shown in

Fig. 7 (b) while the seat was maintained at the upright position within approximately 2 s as shown in Fig. 7(a). The performances for descending step 1 looked similar to step 2, but much lower in terms of the

magnitudes of the torques and tilt angle as can be seen in Fig. 8.

5. CONCLUSIONA wheelchair for ascending and descending staircases has been developed in its simplified form.

The proposed controller has been successfully implemented and tested within simulation exercises. The

results presented has proved the feasibility of the PID control works in controlling highly nonlinear systems

such as a wheelchair on stair climbing process while maintaining the stability of the system. It has been

demonstrated that the control system is able to perform effectively in order to ensure the comfort and

smoothness of the maneuvering tasks.

0 0.5 1 1.5 2 2.5 3-100

-50

0

50

Time (s)

Angularpositionoflink(degr

ee)

0 0.5 1 1.5 2 2.5 3-400

-300

-200

-100

0

100

200

Time (s)

Torque(Nm)

Seat torque

Wheel torque

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ACKNOWLEDGEMENTS

Nor Maniha Abdul Ghani is on her study leave from Faculty of Electrical & Electronics

Engineering, Universiti Malaysia Pahang (UMP) and is sponsored by Ministry of Higher Education

Malaysia.

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[4] Lawn, J.M., Sakai T., Kuroiwa, M and Ishimatsu, T. (2001). Development and practical application of a stairclimbing wheelchair in Nagasaki,InternationalJournal of HWRS-ERC.

[5] Yun. T., Ting. W., Chen. Y. and Xiaofan. L. (2010). The research of tension optimal estimation and stair-climbingability of transformation wheelchair robot, Proceeding of 29thChinese Control Conference, Beijing China.

[6] Morales, R., Feliu, V., and Gonzalez, A. (2010). Optimized obstacle avoidance trajectory generation for areconfigurable staircase climbing wheelchair,Robotics and Autonomous Systems, 58, 97-114.

[7] Sugahara, Y., Ohta, A., Hashimoto, K., Sunazuka, H., Kawase, M., Tanaka, C., Hun-Ok, L. and Takanishi, A.(2005), Walking up and downstairs carrying a human by a biped locomotor with parallel mechanism, Proceeding ofIEEE International Conference on Intelligent Robots and Systems.

[8] Lawn, M. and Takeda, T. (1998). Design of a robotic-hybrid wheelchair for operation in barrier presentenvironments, Proceeding of IEEE International Conference on Medicine and Biology Society, (20), No. 5.

[9] Young, B., Chang-Hyuk., L., Je-Hong, Y. and Kyung-min, L. (2011). Two-legged stair climbing wheelchair and itsstair dimension measurement using distance sensors, Proceeding of 11th International Conference on Control,Automation and Systems, Kintex, Gyeonggi-do, Korea.

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[18]Takahashi, Y., Takagi, T., Kishi, J. and Ishi, Y. (2001). Back and forward moving scheme of front wheel raising forinverse pendulum control wheel chair robot, Proceeding ofIEEE International Conference on Robotics andAutomation, pp.3189-3194.

[19]Takahashi, Y., Ishikawa, N. and Hagiwara, T. (2003). Soft raising and lowering of front wheels for inversependulum control wheel chair robot, Proceeding ofIEEE/RSJ International Conference on Intelligent Robots andSystems (IROS03), pp.3618-3623.

[20]Takahashi, Y., and Kohda, M. (2005a). Human riding experiments on soft front wheel raising of robotic wheelchairwith inverse pendulum control, Proceeding ofIEEE International Conference on Industrial Technology (ICIT05),pp.266-271.

[21]Takahashi, Y., and Tsubouchi, O. (2005b). Modern control approach for robotic wheelchair with inverse pendulumcontrol, Proceeding of 5th International Conference on Intelligent System Design and Application (ISDA 05) ,pp.364-369.

[22]Ding, D and Cooper, R.A (2005). Electric-Powered Wheelchairs: A review of current technology and insight intofuture directions. IEEE Control Systems Magazine.

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